The scanning tunneling microscope has stimulated the development of new techniques for microcharacterization of materials which are based on the use of a very fine tip. One of these techniques involves the atomic force microscope which has recently demonstrated the capability to profile and image conductors and insulators.
In an initial design of the AFM (Binnig G, Quate CF, Gerber Ch, (1986) "Atomic Force Microscope," Phys. Rev. Lett. vol. 56, pp. 930-933 and European patent application Serial No. EP-A-0 223 918) a sensor consisting of a spring-like cantilever which is rigidly mounted at one end and carries at its free end a dielectric tip profiles the surface of an object. The force between the object's surface and the tip deflects the cantilever, and this deflection can be accurately measured, for example, by a second tip which is part of an STM. A lateral spatial resolution of approximately 3 nm has initially been achieved.
Another version of the AFM includes optical detection instead of an STM detection. In this version, a tungsten tip at the end of a wire is mounted on a piezoelectric transducer. The transducer vibrates the tip at a resonance frequency of the wire, which acts as a cantilever, and a laser heterodyne interferometer accurately measures the amplitude of the a. c. vibration. The gradient of the force between the tip and sample modifies the compliance of the lever, hence incucing a change in vibration amplitude due to a shift of the cantilever resonance. Knowing the cantilever characteristics, one can measure the vibration amplitude as a function of the tip-to-sample spacing in order to deduce the gradient of the force, and thus, the force itself. See, Duerig UT, Gimzewski JK, Pohl DW (1986) "Experimental Observation of Forces Acting During Scanning Tunneling Microscopy," Phys. Rev. Lett. vol. 57, pp. 2403-2406; and Martin Y, Williams CC, Wickramasinghe HK (1987) "Atomic Force Microscope-Force Mapping and Profiling on a sub 100-A Scale," J. Appl. Phys. vol. 61(10), pp. 4723-4729.
A critical component in the AFM is the spring-like cantilever. For maximum sensitivity, a maximum deflection for a given force is needed, which calls for a cantilever that is as soft as possible. At the same time, in order to minimize the sensitivity to vibrational noise from the surroundings, a stiff cantilever with a high eigenfrequency is necessary. Ambient vibrations--which typically stem from building vibrations--ordinarily have a frequency spectrum essentially limited to less than roughly 100 Hertz.
If the cantilever is chosen such that it has an eigenfrequency f.sub.o .gtoreq.10 kHz, the ambient vibrations will be attenuated to a negligible value. These requirements can be met by reducing the geometrical dimensions of the cantilever as reflected by the two equations set forth below.
The eigenfrequency f.sub.o of the cantilever is given by ##EQU1## wherein E is Young's modulus of elasticity, .rho. is the density, and K is a correction factor close to unity, l is the length, and t is the thickness of the cantilever.
The spring constant of the cantilever on which its sensitivity depends is given by ##EQU2## wherein F is the force which causes the deflection y of the cantilever, E is Young's modulus of elasticity, w is the width, l is the length, and t is the thickness of the cantilever. In accordance with the spring constant term, the sensitivity of the cantilever is dependent on its dimensions and on the material of which it consists, with the highest sensitivity being obtained for long, thin and narrow cantilever beams. The width of the cantilever beam should be sufficiently large so that lateral vibrations are suppressed. Also, the width of the beam should permit the fabrication of additional structures, such as tips, thereon. Therefore, a minimum width w of around 10 .mu.m is generally required as a practical matter. In practice, the spring constant C has to be greater than about 1 N/m in order to avoid instabilities during sensing of attractive forces, to prevent excessive thermal vibrations of the cantilever beam, and to obtain a measurable response.
Approximates dimensions of a cantilever beam composed of single-crystal silicon compatible with C=1 N/m, and f.sub.o =10 kHz may be, for example: 1=800 .mu.m, w=75 .mu.m, and t=5.5 .mu.m.
In the normal deflection mode of the cantilever beam, forces in the order of 10.sup.-12 N can be detected. The sensitivity of the sensor head can be further enhanced by vibrating the object to be profiled at the resonance frequency f.sub.o of the cantilever beam, as described by G. Binning et al. in Phys. Rev. Lett. vol. 56 (1986), pp. 930-933.
In the AFM realized in accordance with the aforementioned Binning et al. article and with European patent application EP-A-0 223 918, the requirements for cantilever and tip were met by a gold foil of about 25 .mu.m thickness, 800 .mu.m length, and 250 .mu.m width to which a diamond fragment was attached with a small amount of glue. Another proposal used microfabrication techniques to construct thin-film (1.5 .mu.m thick) SiO.sub.2 microcantilevers with very low mass on which miniature cones could be grown by evaporation of material through a very small hole (Albrecht ThR, Quate CF, (1988) "Atomic Resolution with the Atomic Force Microscope on Conductors and Nonconductors," J. Vac. Sci. Technol., pp. 271-274.
From the foregoing description of the state of the art it was known to construct, in a first process step, cantilevers, and, in a second process step, to attach tips thereto. However, the construction of a cantilever with tip of that type is extremely delicate and prone to low yield.
Most recently, several processes have been developed for producing micromechanical sensor heads for AFM/STM profilometry. See, for example, European patent applications EP-A-89 115 100.3, EP-A-89 115 099.7, and EP-A-89 115 097.1.
European patent application EP-A-89 115 100.3 describes a process for producing a cantilever beam with an integrated tip. In this process a mask is produced which contains all relevant information for the desired cantilever beam pattern and tip pattern. In a subsequent etching process the pattern of the mask is transferred step by step to a silicon substrate. The shape of the tip is determined by anisotropic wet and underetching of the tip mask. The tip terminates as a straight line in the form of a multisided pyramid. The cone angle of the tip made in accordance with this process is .gtoreq.35.degree..
However, for the present invention it is desirable to have a method by which conically shaped tips can be produced with cone angles &lt;30.degree., preferably about 10.degree., with highest accuracy and high yield. Particularly in magnetic force microscopy, the domain resolution depends on the tip shape and cone angle.
It is an object of the invention to provide a method for the construction of microcantilevers with integrated tips, which method uses a suitable combination of deposition, lithography, etching, and oxidation steps. The invention also comprises structures made in accordance with the method of the invention.
Prior to starting with the description of the details of the invention, reference is made to the following publications relating to micromechanics:
Petersen, KE, "Dynamic Micromechanics on Silicon: Techniques and Devices," IEEE Tran. Electron Devices, Vol. ED-25, No. 10, October 1978, pp. 1241-1250; PA0 Petersen, KE, "Silicon as a Mechanical Material," Proc. of the IEEE, Vol. 70, No. 5, May 1982, pp. 420-457; and PA0 Jolly, RD, Muller, RS, "Miniature Cantilever Beams Fabricated by Anisotropic Etching of Silicon," J. Electrochem Soc.: Solid-State Science and Technology, December 1980, pp. 2750-2754.